Of these patients, 70 received escalation to 80 Gy (RBE) (13) or 82 Gy (RBE)(52)

Minimum potential follow up was 3 years. Median follow up was 4 years.

Patients were immobilized with placement of Visicoil markers for daily image guidance.

Bladder filling and rectal saline was used and rectal balloons were only used when prostate motion exceeded PTV motion on an as-needed basis.

Target structures and organs-at-risk (OARs) were delineated on CT with aid of MR fusion. MR was found to be particularly useful for prostate apex. Gross tumor volume (GTV) was defined as prostate only in the low-risk protocol and was prostate and seminal vesicles in the intermediate- risk protocol.

GTV was expanded to create the planning tumor volume (PTV) by 8 mm in the superior-inferior axis and 5 mm axially.

The authors identified that the V75 constraint was the most statistically significant.

Author's Conclusions

Outcomes at 3 years with image-guided proton therapy in doses up to 82 Gy (RBE), based on organ-constraints and the techniques used in this study, continue to suggest high efficacy and minimal toxicity.

The correlation noted between grade 2+ rectal bleeding and/proctitis and various dose-volume parameters may be useful in designing dose-volume constraint goals for organs at risk in future clinical trials.

The most powerful dose-volume predictor of grade 2+ GI toxicity is V75, which should be maintained <10-15%. The impact of volume received by low-intermediate doses is controversial; to be prudent, V30 should be less than 40% and V70 less than 20%.

Scientific/Clinical Implications

This study presents one of the first long-term follow up studies assessing the risk of GI toxicity and DVH parameters with proton beam therapy. They show excellent cancer control outcomes and low rates of toxicity.

The DVH parameter that most strongly correlated with GI toxicity is the V75, which is consistent with other studies examining GI toxicity and IMRT. It is of note that this constraint was not empiric at the time of planning. Statistically, it is very challenging to sort out if the V75 alone is responsible for toxicity, or if other low and intermediate doses such as V50 or V30 are also critical as the data is nested within the V75 data. Therefore, we agree with the authors' conclusions that V75, V70, and V30 be minimized in proton treatment planning.

A main strength of this study is that varied gantry angles were used to reduce the topographic variability of the patient.

The rectum was defined in this study from sigmoid flexure to the bottom of the ischial tuberosities. Many institutions use a more stringent definition of the rectum which would make meeting these constraints more challenging.

There are additional strategies that can be employed to decrease PTV margins and therefore spare the rectum further. They include:

Apr 18, 2012 - For patients with nonmetastatic prostate cancer, treatment with intensity-modulated radiation therapy is associated with fewer complications than proton therapy or conformal radiation therapy, according to a study published in the April 18 issue of the Journal of the American Medical Association, a theme issue on comparative effectiveness research.